1. Technical Field
The present invention relates to sensor systems using oscillators, electric circuits used for the sensor systems, and sensor devices equipped with the electric circuits.
2. Related Art
There is a phenomenon in which, when substance included in a surrounding medium adheres to the surface of a crystal vibrator that is in a resonance state, the resonance frequency changes according to the adhered substance. A technology using such a phenomenon is called QCM (Quarts Crystal Microbalance), and used in sensors for detecting the presence or the amount of molecules included in an ambient medium. As application examples of QCM, odor sensors made of vibratos with adsorption films formed thereon for selectively adsorbing specified molecules on the surfaces of the vibrators may be enumerated. Also, applications of QCM to biosensors using hybridization of DNA, gas sensors and the like are under consideration.
In general, AT-cut crystal vibrators are used for QCM devices. The AT-cut refers to a cut substrate in a specific crystal orientation with respect to the quartz crystal axis, and is widely used without being limited to QCM devices because changes in its temperature coefficient adjacent to room temperature are extremely small and it has excellent temperature stability.
An AT-cut crystal vibrator is made of a substrate having excitation electrodes formed on front and back surfaces thereof and operates in a so-called thickness shear vibration mode in which the front surface and the back surface move in opposite directions upon application of a voltage between the electrodes. Its resonance frequency f0 is in reverse proportion to the thickness of the crystal plate at a portion interposed between the front and back electrodes, and generally has the following relation.f0(MHz)=1670/the thickness of crystal plate(μm)
It is known that the relation between changes in the oscillation frequency Δf of the QCM device using such an AT-cut crystal vibrator and the mass ΔM adsorbed on the crystal vibrator is expressed by the following Sauerbrey equation.
                              Δ          ⁢                                          ⁢          f                =                              -                                          2                ×                                  f                  0                  2                                                                              ρ                  ×                  μ                                                              ×                                    Δ              ⁢                                                          ⁢              M                        A                                              [                  Equation          ⁢                                          ⁢          1                ]            
Where f0 is a resonant frequency of the vibrator, ρ is the density of quartz, μ is the shear modulus of quartz, and A is an effective vibration area (i.e., generally equal to the electrode area). It is understood from the equation above that the sensitivity, in other words, the change in the oscillation frequency Δf per the adsorbed mass ΔM can be increased by increasing the resonance frequency f0 of the quartz vibrator.
The above-described quartz vibrator per se relatively excels in temperature stability. However, as changes in the frequency caused by adsorption of substance to be measured by a QCM device are extremely small, small changes in the frequency that may be caused by jitters in the ambient temperature, localized unevenness in the viscosity and density of a solution when a liquid is a measuring environment, and the like, which are generally referred to as drifts, would pose problems.
To correct such drifts, for example, Japanese Laid-open Patent Application 2003-307481 (Patent Document 1) describes a QCM device equipped with a plurality of sensors, which is provided with vibrators with adsorption films formed thereon, and vibrators without adsorption films formed thereon adjacent to the vibrators with the adsorption films. A difference between two frequencies for a test channel and a correction channel is used as a detection value, thereby cancelling drift errors common to both of the channels.
However, the method of cancelling drift errors described above entails some problems. For example, an absolute value of the frequency for each channel needs to be measured, which requires a high measuring accuracy for each channel, whereby a counter circuit as a frequency measuring equipment becomes more complex.